[1] Univ Calif Berkeley, Dept Elect Engn & Comp Sci, Berkeley, CA 94720 USA
[2] Univ Basque Country, Dept Elect & Elect, Leioa 48940, Spain
[3] Univ Basque Country, Dept Immunol Microbiol & Parasitol, Leioa 48940, Spain
[4] Univ Basque Country, SGIKER Magnet Measurements, Leioa 48940, Spain
[5] Basque Ctr Mat Applicat & Nanostruct BCMat, UPV EHU Sci Pk, Leioa 48940, Spain
[6] Lawrence Berkeley Natl Lab, Div Mat Sci, Berkeley, CA 94720 USA